1. Field of the Invention
The present invention relates to electroless plating of silver onto a substrate. More particularly, this invention pertains to an aqueous silver plating bath, a process for plating a uniform coating of silver onto various substrates using an electroless plating composition, and a silver plated article formed therefrom.
2. Description of the Related Art
Plating of metals is a well known process employed to alter the existing surface properties or dimensions of a substrate. For example, a substrate may be plated for decorative purposes, to improve resistance to corrosion or abrasion, or to impart desirable electrical or magnetic properties to a substrate. Plating is a common practice many industries, including the manufacture of a variety of electronic packaging substrates, such as printed circuit boards.
There are various methods of plating known in the art, including electroplating and electroless plating. Electroplating involves the formation of an electrolytic cell wherein a plating metal represents an anode and a substrate represents a cathode, and an external electrical charge is supplied to the cell to facilitate the coating the substrate.
Electroless plating involves the deposition of a metallic coating from an aqueous bath onto a substrate by a controlled chemical reduction reaction which is catalyzed by the metal or alloy being deposited or reduced. This process differs from electroplating in that it requires no external electrical charge. One attractive benefit of electroless plating over electroplating is the ability to plate a substantially uniform metallic coating onto a substrate having an irregular shape. Frequently, electroplating an irregularly shaped substrate produces a coating having non-uniform deposit thicknesses because of varying distances between the cathode and anode of the electrolytic cell. Electroless plating obviates this problem by excluding the electrolytic cell. Another benefit of electroless plating over electroplating is that electroless plating is autocatalytic and continuous once the process is initiated, requiring only occasional replenishment of the aqueous bath. Electroplating requires an electrically conductive cathode and continues only while an electric current is supplied to the cell. Also, electroless coatings are virtually nonporous, which allows for greater corrosion resistance than electroplated substrates.
In general, an electroless plating bath includes water, a water soluble compound containing the metal to be deposited onto a substrate, a complexing agent that prevents chemical reduction of the metal ions in solution while permitting selective chemical reduction on a surface of the substrate, and a chemical reducing agent for the metal ions. Additionally, the plating bath may include a buffer for controlling pH and various optional additives, such as bath stabilizers and surfactants. The composition of a plating bath typically varies based on the particular goals of the plating process. For example, U.S. Pat. No. 6,042,889, teaches an electroless plating bath having a hypophosphite reducing agent and employing one of several different xe2x80x9cmediator ionsxe2x80x9d, including silver ions, for the purpose of converting a non-autocatalytic metal-reduction reaction into an autocatalytic reaction to plate a substrate with copper.
Silver is a desirable plating metal for its high electrical conductivity, corrosion resistance and good friction and wear properties, but present coating techniques are very expensive. Additionally, known efforts to plate silver onto substrates have been imperfect because they use aqueous plating baths containing extremely toxic cyanide compounds and other compounds that contaminate the silver plating.
For example, Japanese patent JP55044540 teaches a process for the electroless plating of silver onto a substrate using an aqueous plating bath comprising silver cyanide, sodium hydroxide and potassium boron hydride as a reducing agent. This bath composition is disadvantageous because of the high toxicity of silver cyanide. Also, it is undesirable because boron hydride derivatives generate extremely flammable gaseous hydrogen and also contaminate the silver metal plating, degrading its appearance.
The present invention solves the problems of the prior art by employing a process for electroless plating of silver using a composition comprising an aqueous solution comprising a water soluble silver salt such as silver nitrate, ammonium hydroxide as a complexing agent, ammonium carbonate and/or bicarbonate as a stabilizer and hydrazine hydrate as a reducing agent. The composition of this aqueous solution is substantially free of non-volatile components that cause impure plating, allowing for improved appearance and properties of the plated silver. Further, the process generates essentially no hazardous substances and the absence of non-volatile components avoids the accumulation of byproducts that degrade the plating bath, allowing for virtually unlimited replenishment of the bath. Moreover, the unique composition of the plating bath allows metallic silver to be precipitated from the plating bath by boiling without undesirable contaminants.
This invention provides a simple low-cost method of a deposition of ultra pure silver coatings on virtually any material of any geometrical shape, including fibers and powders, by electroless autocatalytic plating. The method involves the controlled autocatalytic chemical reduction of a silver salt by a chemical reducer with the formation of a dense uniform metallic silver coating of unlimited thickness selectively on the substrate surface which is contacted with a silver plating bath.
The invention provides an electroless plating composition comprising an aqueous solution comprising:
a) a silver salt;
b) ammonium hydroxide;
c) ammonium carbonate and/or bicarbonate; and
d) hydrazine hydrate.
The invention also provides a process for plating a substrate comprising:
A) providing a plating composition comprising an aqueous solution comprising:
i) a silver salt;
ii) ammonium hydroxide;
iii) ammonium carbonate and/or bicarbonate; and
iv) hydrazine hydrate; and
B) contacting a substrate with the plating composition for a sufficient time and under conditions sufficient to plate metallic silver onto the substrate.
The invention further provides a process for plating a substrate comprising:
A) providing a plating composition comprising an aqueous solution comprising:
i) a silver salt;
ii) ammonium hydroxide;
iii) ammonium carbonate and/or bicarbonate; and
iv) hydrazine hydrate;
B) immersing a substrate into the plating composition for a sufficient time and under conditions sufficient to plate metallic silver onto the substrate; and
C) removing the substrate from the plating composition.
The invention still further comprises an article comprising a substrate immersed in a composition comprising an aqueous solution comprising:
a) a silver salt;
b) ammonium hydroxide;
c) ammonium carbonate and/or bicarbonate; and
d) hydrazine hydrate.
The present invention teaches a process for uniformly plating various substrates with metallic silver using an electroless plating bath. Initially, an aqueous plating bath comprising water, a water soluble silver salt, an ammonium hydroxide complexing agent, an ammonium carbonate and/or bicarbonate stabilizer and a hydrazine reducer is formed in a suitable container.
Once all of the components are combined in a suitable container, the water soluble silver salt dissolves, releasing silver ions into the bath. The ammonium hydroxide complexing agent forms a strong complex with the silver ions and prevents chemical reduction of the silver ions in the bath while permitting selective chemical reduction on a substrate surface. The hydrazine reducer allows reduction of the silver ions to metallic silver which is deposited selectively on a substrate surface because of catalytic action of a substrate surface. In particular, after a substrate is immersed in the plating bath, the substrate surface catalyzes oxidation of the reducing agent. This oxidation causes a release of electrons that, in turn, reduce metal silver ions in the bath at the substrate surface. These reduced metal ions are then deposited onto the substrate and, over time, generate a metal shell around the substrate. The ammonium carbonate and/or bicarbonate stabilizer keeps the plating bath under operable conditions.
Chemical reduction of the silver salt by hydrazine hydrate results in the formation of only metallic silver and highly volatile gaseous byproducts which are removed from the plating bath by mere evaporation. Other bath constituents, including N2H4.H2O, are also highly volatile and can be similarly removed through evaporation. The bath contains no substances capable of accumulating in the container and suppressing the silver plating process, and creates no hazardous substances. The plating composition is highly stable and does not require the addition of non-volatile accelerators, pH regulators or other chemical agents used to enhance plating properties. Also, because no strong complexing agents are included or generated by the bath, simple boiling of the bath is sufficient to precipitate virtually pure silver from the aqueous solution.
This process is autocatalytic, in that no catalyst separate from the aforementioned components is required to advance the silver deposition on a catalytically active surface like base and noble metals, alloys, graphite and others. Catalytically inactive materials like glass, ceramics and polymers can be activated by conventional methods, for instance by contacting with a tin salt solution and/or a noble metal solution. Additionally, the process is continuous and may be maintained for virtually an infinite time by merely replenishing each of the components of the bath.
Following formation of the plating bath, a suitable substrate is immersed in the bath for plating. The substrate remains in the plating solution for a time sufficient and under conditions sufficient to plate a substantially uniform coating of metallic silver onto the substrate. Usually the plating rate is about 0.1 to 2 microns/hour. It increases with increasing temperature and concentration of silver and hydrazine.
The bath is maintained at a temperature ranging from about 20xc2x0 C. to about 98xc2x0 C., more preferably from about 50xc2x0 C. to about 90xc2x0 C. The bath is also maintained at a preferred pH ranging from about 8 to about 13. Preferably, the bath is formed in the absence of any other additives since such would tend to accumulate in the bath. These conditions are important factors in maintaining a stable plating bath and preventing precipitation of silver from the bath.
Typically the substrate remains in the plating bath for from about 1 minute to about four hours depending on the required silver thickness preferably from about 5 minutes to about 60 minutes and most preferably from about 5 minutes to about 30 minutes. After the desired amount of metallic silver has been coated on the substrate, it is removed from the plating solution. The result is an article having a substantially uniform and virtually pure metallic silver plating, having good appearance and properties. Plating can also be done by contacting a substrate surface with a plating bath by any other technique such as spraying, pouring, brushing, etc.
In the preferred embodiment of the invention, the silver salt is water soluble. Such may include silver sulfate, silver chloride and silver nitrate, among others. Of these the most preferred silver salt is silver nitrate (AgNO3). The amount of silver salt present in the bath preferably ranges from about 0.01 to about 650 g/L. More preferably, the amount of silver containing compound present ranges from about 0.1 to about 20 g/L.
The preferred complexing agent is ammonium hydroxide (NH4OH). The most preferred complexing agent is a 28% solution of ammonium hydroxide. Other suitable complexing agents include organic amines, such as methylamine or ethylamine, but these are not preferred. The amount of 28% ammonium hydroxide present in the bath preferably ranges from about 1-1000 mL/L, more preferably from about 10 to about 200 mL/L.
The preferred reducer is a hydrazine compound, most preferably hydrazine hydrate (N2H4.H2O). Other suitable hydrazines include hydrazine chloride and hydrazine sulfate, but are not preferred because of the greater probability that silver will precipitate out of the bath. The preferred amount of hydrazine hydrate present in the bath ranges from about 0.01 to about 210 g/L, more preferably from about 0.1 to about 10 g/L.
The preferred stabilizer is either ammonium carbonate ((NH4)2CO3) and/or ammonium bicarbonate (NH4HCO3). The preferred amount of ammonium carbonate and/or bicarbonate ranges from about 0.01 to about 360 g/L, more preferably from about 10 to about 200 g/L.
Accordingly, the preferred plating bath mechanism can be described by the following general formula:
4 AgNO3+4 NH4OH+N2H4.H2O=4 Ag+N2+4 NH4NO3+5 H2O
The substrate may comprise any material ranging from non-metals, metals, alloys, semiconductors and non-conductors. Suitable metal substrates include stainless steel, carbon steel, nickel, iron, chromium, iron-chromium alloys, and nickel-chromium-iron alloys. Suitable non-metals include printed circuit boards, polyimide substrates, ceramic and glass substrates.
The type of container used to form the plating bath is also an important factor affecting the stability of the bath. In particular, the container should non-metallic to prevent reduction of the metal ions on the walls of the container. Additionally, means used to heat the bath should be a non-metallic heating system, and should heat the bath uniformly to prevent any reductions of metal ions in the bath.
The following non-limiting examples serve to illustrate the invention.